Theory of Intelligent Design, the best explanation of Origins

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Theory of Intelligent Design, the best explanation of Origins » Molecular biology of the cell » Cell Gatekeepers: Diverse, Complex, Accurate

Cell Gatekeepers: Diverse, Complex, Accurate

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1 Cell Gatekeepers: Diverse, Complex, Accurate on Sun Jul 26, 2015 7:07 am


Cell Gatekeepers: Diverse, Complex, Accurate 2
Cargo moves around rapidly and ceaselessly in every cell.  Some moves in and out of the external membrane, and some moves in and out of organelles and the nucleus.  In a system of protected domains surrounded by impermeable membranes, how does the cell control what should pass?  Details of the amazing gatekeeping mechanisms embedded in cell membranes have been coming to light for years now.  Some recent articles have reported the latest findings.

Protective sleeve:  One method of getting valid cargo through the membrane gate is to wrap it in a protective sleeve that the gate recognizes.   Purdue university has an illustration from the work of a team at Purdue showing how this works.

A research team led by Jue Chen obtained a snapshot of the tiny protein gate complex that opens and closes pathways through the protective cellular membrane. The gates, operated by small protein machines that push them open and closed, bring nutrients into the cell and flush out waste.

The Purdue-led team was the first to achieve an image of the middle step of the process, capturing the molecular interactions as material passes through the membrane.

"By understanding the mechanisms of this process, researchers may be able to design more effective treatments for diseases that involve this group of proteins, such as cancer and cystic fibrosis," said Chen. "With this knowledge, researchers may be able to inhibit or activate this mechanism, depending on what is needed to counteract the disease. For instance, many cancer cells are resistant to drug treatments because the cells pump the drugs out through these channels before they can work."

The research team used X-ray crystallography to obtain a picture of a special protein, called an ABC transporter protein, as it moved material through the cellular membrane. The work was published in last week's issue of Nature.

 What comes to mind is a personal subway capsule that shuttles you to an escalator that transfers you safely into a shopping mall without any intruders getting past.

Electronic gating:  Ions are electrically-charged atoms whose concentration in the cell must be strictly controlled.  Compared to the large molecules of the cell, ions of potassium, chlorine and sodium are tiny.  Special voltage-sensing gates exist just for them.  We reported here on early results from work by Roderick MacKinnon into the structure and function of these ionic gates (see 01/17/2002, 05/29/2002, 05/01/2003, 08/05/2005).
   The November issue of The Scientist describes ongoing discoveries about one of these voltage-gated channels, the Kv potassium channel.  This electronic mechanism contains a pore, a gate and a voltage sensor.  In particular, a key helix protein component called S4 undergoes a conformational change to open the gate for the potassium ion.  People who enjoy exercise may want to reflect that all nerve and muscle activity depends on the proper control of these ions.
Nuclear power plant security:  For those wanting to follow up on news about the nucleus, and how it controls the cargo going in and out (see last month’s entry, 11/13/2007, bullet #2), the crew of your nuclear power plant made the cover of Science this week.  Laura Trinkle-Mulcahy and Angus I. Lamond reviewed the latest work to get high-resolution images of the complex structures and functions of the nuclear membrane, especially the gates of the nuclear pore complex (NPC).1
   Four other articles in the 11/30 issue describe the latest findings about the cell nucleus.  A paper by 3 Vanderbilt University scientists specifically addresses the factors involved in crossing the nuclear envelope through the NPC gates.2  For those wanting more information about the sensing mechanism, their article contained color diagrams of the structures.  The scientists explained how the gates are regulated at multiple levels – a philosophy common in national security and computer security, too.  The “dynamic and diverse” mechanisms control what passes at the gate level, the transport receptor level, and the cargo level.  In computer parlance, this might be analogous to requiring a fingerprint, a secure computer, and secure software before you are allowed to login.
   Another paper in the same issue of Science describes science’s growing realization that the nuclear membrane does far more than let things in and out.3  It is actively involved in cell division, structuring the cytoskeleton, and signaling other processes in the cell.  The nuclear envelope is also connected to the endoplasmic reticulum, a structure essential for post-translational modification of proteins.  The authors did not mention how these elaborate mechanisms might have evolved, except to say twice that they raise “intriguing questions” and “fundamental questions” about “evolutionary relations” between the parts.  The other two papers did not mention evolution at all.
ER: emergency room or endoplasmic reticulum:  Speaking of the endoplasmic reticulum (a kind of subway system within the cell), Nature reported studies about the transport channels in that organelle.4  “A decisive step in the biosynthesis of many proteins is their partial or complete translocation across the eukaryotic endoplasmic reticulum membrane or the prokaryotic plasma membrane,” began Tom Rapoport (Howard Hughes Medical Institute, Harvard).  “Most of these proteins are translocated through a protein-conducting channel that is formed by a conserved, heterotrimeric membrane-protein complex, the Sec61 or SecY complex.”
   Polypeptides are the pre-protein strings of amino acids emerging from ribosomes, where the translation from RNA occurs.  Getting a wobbly chain of molecules through a pore is somewhat akin to threading a needle.  Depending on what the cargo binds to, it may get in by one of several ways: the ribosome may simply attach to and inject the nascent polypeptide into the channel, an ER chaperone might pump it in by a ratcheting mechanism, or a molecular machine running on ATP might push the polypeptide through.  These are all regulated by a host of assisting proteins that keep in touch through signaling mechanisms.  There’s even a plug that closes the channel after the polypeptide is inside.
   Rapoport provided a diagram of the complicated-looking translocation channel, which is made up of three different protein parts.  He called it conserved (unevolved) between all three kingdoms of life, but did not say anything else about evolution – certainly, not anything about how it arose in the first place.
Light sensitive:  Imagine a receptor on a cell membrane that can respond to one photon of light, and send a signal into the interior.  You don’t have to imagine it: it already exists.  Rama Ranganathan in Science described the family of G-protein coupled receptors (GPCR) that “occur in nearly every eukaryotic cell and can sense photons, cations, small molecules, peptides, and proteins.”5  How do they do it?  The structures of these receptors are just beginning to come to light, and basic models are being formulated.  Stay tuned.


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